Wood fuelled heating stove

ABSTRACT

A wood fuelled heating stove including: a stove portion having a stove chamber therein and an fine mesh charcoal basket dividing the stove chamber into upper and lower chambers; a reduction portion having a primary combustion chamber therein and at least partly within the upper chamber of the stove chamber and having a primary grating vertically separating the primary combustion chamber from the upper chamber; an air jacket surrounding at least some of the primary combustion chamber and having a plurality of ports providing fluid communication between the primary combustion chamber and the air jacket and a primary grating providing limited flow of mobile solids and fluid between the primary combustion chamber and the stove chamber; an air passage providing fluid communication from the outside of the stove chamber to the air jacket and to the inside of the stove chamber; and a flue passage providing fluid communication from the lower chamber of the stove chamber to a flue outlet at a level above the primary combustion chamber and upper level of the stove chamber.

This invention relates to a wood fuelled heating stove. The inventionhas particular application to a wood fuelled heating stove which uses“reverse flow progressive oxidation” such as that described in myAustralian Provisional Patent Application Nos. 2012904682 filed 26 Oct.2012 and 2013901350 filed 17 Apr. 2013, the contents of which areincorporated herein by reference. However, the invention is notnecessarily limited to such stoves nor necessarily to the theoreticaloperation described in the specifications of the aforesaid applications.

A basic open fire has an ash zone at the base above which the wood is“baked” to release carbon-based gases which ignite when mixed with aircoming in from the sides. The hot gases rise to heat newly added woodfuel. Cool air flows in from the sides, starting near the base butcontinuing up the sides. Fire places and wood fuel burning stoves havethis basic open fire structure. The intrinsic problem with this kind ofstructure is that the physical movement of the gaseous fuels and theinflow of cooler air is conducive to the production of unburnt carbonparticles, tars and creosotes, some of which may become air pollutants.The highest average temperatures are located in the burning charcoalregion of the open fire as well as in prior art stoves. The flaminggases above are from the wood above releasing hydrocarbon and othercarbon-based gases which undergo some combustion with inflowing coolerair. More readily reactive hydrocarbons tend to be those of the lowerdensities and lower vaporising temperatures, the combination of whichsometimes leaves tars and creosotes as unburnt air pollutants in somecircumstances and often also producing undesirable smoke.

Smoky fires can result from insufficient heating of the air passingthrough the bed of coals, particularly where a bed of coals has not beenestablished, or where the amount of new fuel exceeds what can be heatedand burned with little or no smoke. In a stable wood fire, some of theoxygen content of the air is used in the combustion of carbonaceousmaterial in the coals with consequential heating of the air to atemperature sufficient to ignite part of the wood fuel above the bed ofcoals. Traditional wood fuelled heating stoves often do not completelycombust the wood or combustion can be inefficient. Standard enclosedstoves crudely direct air to the general area of wood placement with upto 50% of the air not used in the combustion process and effectivelycooling the combusted gases which reduces the incidence of furthercombustion.

The present invention aims to provide a wood fuelled heating stove whichaddresses one or more of the shortcomings of the prior art. Other aimsand advantages of the invention may become apparent from the followingdescription.

With the foregoing in view, in one aspect the present invention residesbroadly in a wood fuelled heating stove including:

a stove chamber having a floor, a side wall extending upwardly from theperiphery of the floor and a top wall extending across the top of theside wall;

a primary combustion chamber at least a part of which extends at leastpart way into the stove chamber through an opening penetrating the topwall, the primary combustion chamber having a base wall interposedbetween the floor and the top wall of the stove chamber and having aprimary grating provided comprising at least a part of the base wall, acurtain wall extending upwardly from the base wall through the openingin the top wall and having a plurality of ports penetrating the curtainwall and a closure wall extending across the top of the curtain wall toenclose the primary combustion chamber;

an air jacket wall circumscribing at least part of the curtain wall atleast partly below the opening to provide an air jacket circumscribingthe primary combustion chamber but separated therefrom by the curtainwall, the curtain wall having a plurality of ports distributed about andpenetrating therethrough to provide fluid communication between the airjacket and the primary combustion chamber, the air jacket wall and/orthe curtain wall substantially sealing the opening through the top walland separating the air jacket from the stove chamber;

an air inlet opening through the floor or the side wall at or near thefloor and a stove chamber air inlet into the stove chamber through ornear to the top wall;

a primary air duct, a secondary air duct and a tertiary air duct, theprimary air duct extending along the floor of the stove chamber andproviding fluid communication from the air inlet to the secondary airduct, the secondary air duct extending upwardly from the primary airduct and the tertiary air duct providing fluid communication between thesecondary air duct and the air jacket and the stove chamber air inlet;

a charcoal grating partition interposed between the floor of the stovechamber and the base wall of the primary combustion chamber; and

a flue passage extending from a flue opening to the stove chamber lowerthan the grating wall and upwardly to a flue outlet higher than the topand closure walls.

In another aspect, the present invention resides broadly in a woodfuelled heating stove including:

a stove portion having a stove chamber therein and an fine mesh charcoalbasket dividing the stove chamber into upper and lower chambers;

a reduction portion having a primary combustion chamber therein and atleast partly within the upper chamber of the stove chamber and having aprimary grating vertically separating the primary combustion chamberfrom the upper chamber;

an air jacket surrounding at least some of the primary combustionchamber and having a plurality of ports providing fluid communicationbetween the primary combustion chamber and the air jacket and a primarygrating providing limited flow of mobile solids and fluid between theprimary combustion chamber and the stove chamber;

an air passage providing fluid communication from the outside of thestove chamber to the air jacket and to the inside of the stove chamber;and

a flue passage providing fluid communication from the lower chamber ofthe stove chamber to a flue outlet at a level above the primarycombustion chamber and upper level of the stove chamber.

In such form, a portion of the grating wall comprises a grating throughwhich ash or such like may fall to rest upon the floor of the stovechamber. Preferably, the side wall includes four side wall portions,each substantially planar which, together with the floor and top wall,provide a substantially rectangular prism form for the outside walls ofthe stove chamber. Preferably, one of the side wall portions has a dooropening through and a the door being moveable between an open attitudepermitting the access to the stove chamber and a closed attitudesubstantially sealing the stove chamber against air flow through thedoor opening as well as sealing the upper chamber from the lowerchamber.

Preferably, the primary combustion chamber is of similar form to that ofthe stove chamber, but is smaller in size, and arranged to dependdownward from the top wall. In similar fashion, the air jacket surroundsthe lower portion of the primary combustion chamber in rectangular prismform, part of the air chamber wall provided as an extension of the basewall or extending outwardly from the chamber wall. In such form, the airjacket wall extends upwardly from its attachment to the base wall orcurtain wall to the inside edges of the opening through the top wall ofthe stove chamber. Alternatively, the air jacket wall ends short of thetop wall and rejoins the curtain wall. In such form, the chamber wallprovides the sealing arrangement about the inner edge or edges of theopening through the top wall.

For convenience, the side wall portion with the door opening will bereferred to as the front wall and, observing the stove chamber from theoutside of the front wall, a right side wall is to the right and a leftside wall is to the left of the front wall, and a rear wall is oppositethe front wall adjoining the right and left side walls. Preferably, theair inlet to the primary air duct is provided substantially centrallythrough the front wall of the stove below the door. In such form, theprimary air duct extends horizontally along the floor and front of thestove chamber to the secondary air duct, which comprises two risingducts which run along each side of the front wall from the ends of theprimary air duct. Preferably, the primary air duct has a damper or suchlike interposed between the air inlet and the secondary air duct fordamping or restricting airflow through the primary air duct. Preferably,the air inlet forms an orifice to the primary air duct and the damper isprovided to selectively occlude, or partly occlude, the orifice.

Preferably, the tertiary air duct provides fluid communication betweenthe two secondary air ducts, though in operation, air flow through thesecondary air ducts is substantially parallel, flow from the primary airduct being split between the two secondary air ducts and joiningtogether again in the tertiary air duct. However, the tertiary air ductsplits the air flow again, some of the air passing to and through thestove chamber inlet and the remainder of the air passing to and throughthe air jacket.

The plurality of ports are arranged to provide limited air flow to theprimary combustion chamber, but the majority of the pressure dropthereto. The remainder of the air flow is directed to the stove chamber.The air flowing to the primary combustion chamber in use is at leastpartly deoxygenated with the combustion of wood fuel and hot coals inthe primary combustion chamber to produce hot gas, but the draft of hotgases from combustion is downward through the grates and then upwardthrough the flue passage by virtue of the enthalpic rise of lowerdensity hot gases in the flue passage.

The present invention relates to a reverse flow, progressive oxidationwood fuel heating stove encompassing an improved combustion processwhich may be effected in the stove herein described. The oxidation ofwood fuel in the stove is progressive, the wood moving by gravity tolocations of increasing oxidation with a gas and flame flow which is thereverse of that which may be regarded as “normal”.

It has surprisingly been discovered that if the fire gases are made toflow downward, combustion of the problem hydrocarbons and carbonparticles occurs more readily because the vertical temperature gradientmay also be reversed. Sustaining the flow of air for the reverse flowmovement is by way of the chimney draft. The natural tendency for air torise through the fire is overcome by the strength of the downward draftwhich is enhanced by locating the chimney uptake close to the hot gasesexiting the tertiary burning charcoal and the chimney or flue outletwell above the level of the fire. This also permits the chimney to beused more effectively as a heat radiator.

The progressive oxidation has been tested and observed to operate in anenclosed box (the primary combustion chamber), open only at the base andproviding a combustion pathway of length sufficient to allow theoxidation processes to occur in an ordered way. Air intake ports ofpredetermined size are strategically located to penetrate the sides ofthe primary combustion chamber. The size and location of the air portsare selected to control air without a need to use the damper as well asto introduce a kinetic or momentum component to the air flow to create,or at least to encourage the creation of, turbulence.

When the system is combusting and the draft flowing, some first air,limited in amount, is drawn through the upper holes for an initial lowertemperature burning, which progressively heats the wood fuel lower downand preheats the wood above by radiation. As more air enters lower down,the air becomes at least partly depleted of oxygen with progressivelyhigher temperatures. Because the temperatures are higher at the base,final disintegration of the wood can occur at the base which createsspace for the upper level to progressively move down to the nextoxidation position.

A wood feed door is provided at the front top of the primary combustionchamber with a rotating locking handle on the side. A locking handle iscoupled to a chimney bypass door at the rear by a rod. The lower portionof the primary combustion chamber has a predetermined number ports ofpredetermined size. This portion of the primary combustion chamber isenclosed by an air jacket.

In an alternative form, a basket is suspended at the open end of theprimary combustion chamber. Grating rods extend across the bottom of theprimary combustion chamber and the grating basket is suspendedinternally by vertical rods. The primary combustion chamber may beprovided as a primary combustion chamber module fitted through the topof the main stove body which may be specially constructed, or may be anexisting wood fuelled stove body of the prior art converted to a woodfuelled heating stove according to the invention.

In operation ash falls through the recessed fine mesh charcoal basketinto the lower chamber, which acts as a “final flaming area” where thenewly introduced oxygen from the air curtain flow is mixed with theoxygen depleted gases exhausting the primary combustion chamber and haspassed through the flaming charcoal in the fine mesh charcoal basket.The lower chamber also leads to the start of a vertical exhaust boxchamber which forms a lower portion of the chimney. A chimney bypassdoor can be rotated or otherwise moved across the exhaust box chamber toblock off or reduce normal chimney flow, but open flow to the chimneyfrom the upper end of the reduction chamber. The other end of thevertical exhaust box chamber receives the vertical portion of chimney.

In another aspect, the present invention resides in an improvedcombustion process for domestic wood heating stoves using a form ofstratified downdraft where the oxidation of the wood fuel is progressivewith the vertical movement of the wood feed starting with an oxygenlimited combustion and progressing to combustion which is not so limitedat the base.

This invention has two advantages that result in higher stovetemperatures per unit of wood burnt. Firstly, it is able to combust highyielding fuels such as tars, creosotes and carbon particles thatstandard stoves often fail to burn. Secondly, the invention uses asystem of limited, but specifically targeted air. This approachincreases the stove temperature because unused cooler air does notunnecessarily increase total gas volume or cool the combusted gases. Thecombination of these two advantages results in a stove that operates ata temperature which, based on testing conducted by the inventor, is upto 30% higher without any increase in wood consumption.

In order that the invention may be more readily understood and put intopractical effect, the basic open fire structure of the prior art and anexample of the present invention will now be described with reference tothe following drawings, and wherein:

FIG. 1 is a diagrammatic side elevation of a basic open fire structureaccording to the prior art;

FIG. 2 is a pictorial view of a wood fuelled heating stove according tothe invention;

FIG. 3 is a vertical cross section of the stove of FIG. 2 along line 2-2of FIG. 5;

FIG. 4 is a horizontal cross section of the stove of FIG. 2 along line3-3 of FIG. 5;

FIG. 5 is a vertical cross section of the stove of FIG. 2 along line 4-4of FIG. 4;

FIG. 6 is a diagrammatic oblique view of part of a grate for the stoveof FIG. 2;

FIG. 7 is a diagrammatic exploded view of some of the parts of the grateof FIG. 5; and

FIG. 8 is a diagrammatic pictorial view of part of an adjustment systemfor the grate of FIGS. 6 and 7;

FIG. 9 is diagrammatic sectional side view of part of the combustionprocess in the primary combustion chamber of the stove of FIG. 2;

FIG. 10 is a diagrammatic sectional side view of the reduction portionof FIG. 9 showing its disposition to with respect to the stove portion;

FIG. 11 is a diagrammatic sectional side view of the reduction portionof FIG. 9 showing its disposition with respect to the flue;

FIG. 12 is a diagrammatic sectional front view of the reduction portionof FIG. 9 showing an optional basket for raising and lowering fuel inthe reduction portion;

FIGS. 13 and 14 are diagrammatic sectional side view of the stove ofFIG. 2 with the upper portion of the flue removed;

FIG. 15 is another diagrammatic sectional side view of the stove of FIG.2;

FIG. 16 is a diagrammatic front elevation of part of the flue for thestove of FIG. 2; and

FIG. 17 is a graphical representation comparing combustion temperaturesof an open fire of the prior art with combustion temperatures of a firein the stove of FIG. 2.

In FIGS. 2 to 16 of the drawings, because different parts or portions ofthe wood fuelled stove are exemplified in different drawings, not all ofthe elements are shown in all of the drawings and consequently, some ofthe reference numerals are provided in some of the drawings.Additionally, some of the description will refer to the way the stoveoperates when it is in use.

A basic open fire structure can be divided into zones “A”, “B”, “C” and“D” for convenience of explanation. At the base of the fire is an ashzone “A” containing ash and charcoal. Burning of the charcoal to ash“bakes” the wood above to release carbon-based gases which ignite whenmixed with air in zone “B” which contains flames, burning gas and bakingwood. The flames stream upwards to heat newly added wood fuel in zone“C” which also contains released gases for burning to produce the flamesabove in zone “D”. Air flows in from the sides, starting near the baseof the fire and also up the sides, the inflowing air being cool, thatis, at or not much above ambient temperature.

The wood fuelled heating stove 10 illustrated in FIGS. 2 to 5 includes astove portion 11 and a reduction portion 12 partly penetrating into thestove portion and partly extending upwardly therefrom. The stove portionis supported on a plinth or base 13. The stove portion has a floor 14, aright side wall 15, a rear wall 16, a left side wall 17, a front wall 18and a top wall 27 which enclose a stove chamber 41, the reductionportion having enclosed therein a reduction chamber 42. The front wallhas a door 19 providing access to the stove chamber. It can be seen thatwhen closed the door seals against the partition at 19 a.

An air inlet 20 proceed from the front wall and leads to a primary airduct 22. The primary air duct is a horizontal duct extendingtransversely along the front bottom corner of the stove chamber. Tworising secondary air ducts 21 and 23 extend from the ends of the primaryair duct and extend upwardly along the front left and right corners ofthe stove chamber to a tertiary air duct 24 above the top wall of thestove chamber.

The tertiary air duct provides fluid communication from the secondaryair duct to an air jacket 26. Once stable combustion is established inthe wood fuelled heating stove, inlet air passes through the inletrepresented by arrows 29. The inlet air passes through the horizontalportion of the primary air duct and into the secondary air ducts asrepresented by arrows 30. The inlet air then passes upwardly through thesecondary air ducts and into the tertiary air duct has represented byarrows 31.

Some of the air passing through the tertiary air duct passes through astove chamber inlet 40 into the stove chamber as represented by arrows32. The remainder of the air passing through the tertiary air ductpasses therethrough as represented by arrows 33 and 34. For convenience,the air passing from the tertiary air duct to the air jacket will bereferred to as primary combustion air. The primary combustion air passesfrom the tertiary air duct to the air jacket, proceeding around the airjacket as represented by arrows 35 and 36 including downwardlythroughout the air jacket as represented by arrows 37. The primarycombustion air from the air jacket passes through a plurality ofairports 39 and into the primary combustion chamber as represented byarrows 38.

The airports are provided in a curtain wall 51 which surrounds thereduction chamber. The air jacket is separated from the stove chamber bya curtain wall 50. The stove chamber is divided into an upper stovechamber 54 and a lower stove chamber 55 by a charcoal grating partition52 having a fine mesh charcoal basket 53 disposed somewhat centrally inthe charcoal grating partition. The reduction chamber is separated fromthe upper stove chamber by a primary grating 56 having a plurality ofopenings 57 through which hot coals may fall to land on the fine meshcharcoal basket for combustion to an ash.

The primary grating has a variable aperture or spacing as illustrated inmore detail in FIGS. 6 to 8. The primary grating includes two sidechannel sections 58 which are arranged opposite one another with thechannels facing towards one another. Only one of the side channelsections is illustrated in FIGS. 6 and 7, but the arrangement issubstantially symmetrical. Two sets of bars extend between the opposedside channels with the ends of each bar inserted at least part way intothe channel of each side channel section.

Each set of bars includes a plurality of bars shown typically at 61substantially parallel to one another within each set as well as thebars of one set being substantially parallel to the bars of the otherset. One set is a fixed set 59 and the other set is a moveable set 60which may be slid along the channel, the direction of sliding beingalong the channels and transverse to the bars. Each set of bars hassubstantially the same spacing.

The fixed set is fixed to the side channel sections by welds 62 at ornear the ends of each fixed rod and at regularly spaced intervals alongthe lips 58 a of the side channel sections. The ends of the bars of themoveable set of bars are fastened in spaced disposition to a sliding bar63 which has cross-sectional dimensions selected for sliding engagementwith the channel of the side channel sections. The ends of the fixedbars are shorter than the moveable bars so that the sliding bar canslide past the ends of the fixed bars. The moveable bars are also fixedto a push-pull bar 64, a portion of which extends away from theremainder of the moveable bars to a connecting rod 65 and the remainderis welded to at least one, but preferably all, of the moveable bars bywelds at 73.

The connecting rod is pivotally connected to a lever portion of a crankassembly 67. The lever portion is fixed to a torsion rod 68 having anaxis parallel to the axis of the pivot to which the connecting rod isconnected. A crank portion 69 extends at rights angles to the torsionrod and has a crank handle 70 attached or fastened thereto. Rotation ofthe torsion rod in the direction of a rotational arrow 71 by applicationof a tangential force to the crank handle causes linear movement of theconnecting rod and push-pull bar, whereupon the moveable rods are movedin the direction of a linear arrow 72. It can be seen that therotational and linear arrows are bi-directional, indicating that themoveable rods may be moved from against one side of the fixed bars toagainst the other side of the fixed bars, and at any point therebetween.

In the diagrammatic sectional side view of FIG. 9, new wood at 111 isadded at the top and as it falls through the chamber, undergoes limitedburning at 112, burning at 113 and intense burning at 114. At 115, thewood has been converted to charcoal and is flaming charcoal at 115. Airis introduced, as indicated by arrows at 116, but in a limited amount inaccordance with the invention. The graph to the right of thediagrammatic sectional side view illustrates the relationship betweenthe oxygen availability for combustion and the temperature, also withrespect to the relative position in the combustion chamber.

The reverse combustion process will now be described with reference toFIGS. 10 to 15. To attain control of the reverse flow fire process,stove according to the present invention is constructed using two basicmodules. The first module is the primary combustion chamber constitutingthe reduction portion 12, which is enclosed at the top and sides butopen at the bottom. The primary combustion chamber uses the primarygrating 56 at the bottom to retain the feed wood 43 therein. The secondmodule is the stove portion, and is substantially in the form of acommon wood combustion stove which has been modified to include theprimary combustion chamber half set into the roof of the stove. The fluereceives gases only from the lower stove chamber.

It is suggested that the reverse flow fire process has threefoldelements: Firstly, suction is provided by the draft from the flue,thereby enabling the down draft through the combustion module into theupper stove chamber, through the fine mesh charcoal grating into thelower stove chamber with the exhaust gases exiting via the flue.

Secondly, splitting the air and targeting the major portion thereof tothe combustion chamber and the remainder to the upper stove chamberresults in higher temperatures being concentrated in smaller volumes ofgases that move more slowly through the system, thereby providingpre-heating of air before entering the combusting areas of the stove andmaximising the oxidation of the wood fuel in the primary combustionchamber. Air is directed around the sides of the stove body to use theheat radiating from the sides of the stove to pre-heat the air used forcombustion of the wood and associated volatile gases, tars andcreosotes. Air flowing through the lower air ports flames the charcoalon the coarse mesh grate used and enables secondary combustion ofunburnt gases and also helps with the tertiary combustion of anyremaining volatile gases passing through the charcoal on the fine meshcharcoal basket.

The primary combustion chamber is set into the top wall of the stoveportion such that approximately half of the primary combustion chamberis positioned above the stove roof, while the lower half extends intothe upper portion inside the stove. To assist in safe operation, thefire is dampened when the adding additional wood by constricting orclosing off the draft through the chimney by a connecting a linkage andhinge that connects the wood feed access door locking handle to thechimney diversion door.

Within the upper stove chamber, the primary combustion chamber airmanifold or air jacket surrounds the lower half of the primarycombustion chamber receives pre-heated air and further heats air fromthe air supply system. The pre-heated air is drawn in such volumes thatoxygen limited burning in the upper part of the primary combustionchamber progresses to oxygen rich burning in the lower part.

In an alternative form, a basket is provided which can be raised orlowered within the primary combustion chamber to alter the combustion ofthe wood fuel and its relation to the relative position of the air entryholes. The height of the basket is set by a hollow course pitch screwthread which is rotated by a crank handle. A rod passes through thehollow screw, then passes through the length of the combustion chamberand is attached to the centre of a blade which can be rotated by ahandle to break up and agitate the large pieces of charcoal sitting onthe grate. A spring placed between the handle and the top of the hollowscrew positions the blade or stirrer in a raised position. The rodextends through locating holes in the grate and terminates with a secondstirring blade, the rotation of which, and height variation, is used tosmooth and distribute charcoal that has dropped through the coarsegrating.

The partition between the upper stove chamber and lower stove chamberhas soft, but airtight seal against the stove access door. The fine meshcharcoal basket is recessed down into the partition to (receive charcoalfalling from the course mesh charcoal grate at the base of thecombustion chamber. The flue may be is split as shown in FIG. 16 intotwo (or more) parallel pipes to increase the radiating surface area.

Being a reverse flow (downdraft) fire, a downdraft is required beforethe main fuel body can be ignited. To achieve this the followingprocedure is required:

The Chimney is preheated by igniting a tray of commercial alcohol (40-50ml) under the chimney opening in the lower stove chamber to allow thechimney to be heated to about 100° C. in approximately 40 seconds.

With the main stove access door open, the charcoal and kindling in thefine mesh charcoal tray separating the upper and lower stove chambers islit, access thereto being via the stove body door which is closed afterthe lighting operation. The charcoal strongly burns within 60 secondswith a chimney temperature of about 150° C. and rising with a strongreverse flow through the stove.

After the burning of charcoal in the fine mesh charcoal tray hasstabilised, the air supply system control is opened. Two minutes afterlighting the charcoal in the fine mesh charcoal tray the chimneytemperature reaches approximately 200° C. and a strong downdraft isevident.

Using kindling on top, a moderate volume of the main feed wood in theprimary combustion chamber is lit, with the primary combustion wood feedaccess door open, and the air supply system open. As the wood catches onfire and combustion stabilises, the wood feed access door is graduallyclosed and the air supply system is adjusted to optimise combustion.

After the feed wood has been ignited in the base of the combustionchamber, the volume of wood in the combustion chamber is graduallyincreased to give time for charcoal to form on the course mesh charcoalgrate and the fine mesh charcoal basket enabling secondary and tertiarycombustion. All air in use in the primary combustion chamber and theupper stove chamber is supplied from and controlled by the air supplysystem. With the ignition operating in a stabilised state, the preheatedair delivered for the air curtain on the inside face of the stove bodydoor is of the order of 200° C., and in the in the primary combustionchamber air manifold entering the primary combustion chamber at about250° C. at the top and about 300° C. at the bottom. By partiallycapturing the heat radiating from the stove body and circulating thisheat back into the primary combustion chamber via the air supply systemand the primary combustion chamber air manifold, the temperature of theunburnt wood is raised which assists in the removal of moisture thusenabling the primary combustion of the wood and the release of volatilegases.

In FIG. 14, the progressive oxidisation of the feed wood begins at thelevel X1 of the first air inlet. At this level the air is limited andsupports only weak oxygen-limited combustion. The movement of gas issubject to the strong draft and the overall movement is downwards,accelerating as more air inlet holes are passed, more fuel combusted,the temperature rises and the combustion process progressively changesfrom oxygen limited at X1 to become less oxygen limited at X2.

At about this point the hydrocarbons have mostly been subject topyrolysis to be cooked out of the original wood feed to leave charcoalas the dominant residue. In an oxygen rich environment the charcoal atX3 is the dominant fuel with a temperature in excess of about 1,000° C.This is sufficient to combust all of the hydrocarbons provided thatoxygen has been able to mix from the sides and past obstacles at thebase. The combusting charcoal reduces in size, fractures with the weightabove or by mechanical crushing. Charcoal then falls through the grateto replenish the charcoal in the fine mesh charcoal basket or tray.

The progressive oxidation of the system within the primary combustionchamber requires a column of wood to be baking and releasing gases abovethe lower oxygen rich combusting area. If the wood is withheld with thethought of slowing the combusting rate, the fire will tend towards beinga common fire that happens to be downdraft, the lower charcoal beds willdiminish, combustion will be incomplete and the process will deterioratewith pollution deposits internally and externally.

Additional air at X4 from the air curtain is added and drawn through thesmaller charcoal in the basket. This action of drawing gases through thebasket containing smaller charcoal pieces creates extra mixing as itpasses through and the extra air flames the charcoal. The gases emerginginto lower stove chamber are different, produce a flames of a differentcolour and have been all but purged of tars and creosotes as can be seenby the lack of condensate deposits on the lower stove glass door whichhas no protective air curtain.

Ash falls to the floor, the gases are drawn down and over the ash pileand the combusted gases are drawn up the rear flue space to egress intoa large diameter thin walled chimney flue. To slow down the velocity ofthe flue gases the flue is duplicated by a union. This configurationsignificantly increases the time for heat energy to be radiated anddistributed by convection into the domestic space before being releasedinto the atmosphere. The stripping of heat can be done without adverselyaffecting the workings of the stove because of the high temperatures ofthe gases at the base of the chimney (approximately 500° C.).

It has been found that the wood fuelled heating stove according to thepresent invention when used in the manner described herein arranges forboth the depleted wood solids and various gases of the process to arriveat the final oxidation position at the highest temperature of theprocess and in an environment of controlled availability of oxygen. Thewood and air proceed in the Same direction downwards, the wood bygravity and the air by the chimney draft. It is suggested that chimneytemperature be monitored and maintained as it provides the driving forceof the reverse flow action.

Referring to the graph in FIG. 17, in a conventional fire the volatilegases released in the primary combustion rise vertically from theburning charcoal, not through it. As these flaming gases rise they coolwhile′ mixing with inflowing air. The resulting cooled air/gasstoichiometric mixture restricts the maximum temperature at the top ofthe flame to approximately 300° C. as represented by line 101 in FIG.17. The available oxygen or the temperature are often insufficient tocomplete the secondary combustion of the volatile gases and particlesreleased from the burning wood when mixed with air.

In the reverse flow system of the present invention, the combustiontemperatures of the gas mixture increase as the flow moves downwardsculminating with the gases passing through the hottest area of the fireburning charcoal at approximately 1,000° C. as represented by line 102in FIG. 17. The gases released from the burning wood reach their maximumtemperature just prior to exiting the system as exhaust gases. Mixing anadditional and pre-heated oxygen supply to where the gas has reached itsmaximum temperature creates optimum conditions for the tertiarycombustion of remaining combustible gases and particles as an additionalfuel source. This enables the near complete combustion of the wood andall by-product gases expelled during the primary combustion phase,thereby increasing the efficiency of stove while also reducing thequantity and nature of the pollutants finally released through thechimney flue.

Primary combustion takes place where the wood is burned in both anoxygen limited and an oxygen rich environment in the primary combustionchamber. Secondary combustion takes place in the charcoal resting on thecoarse mesh charcoal grate at the base of the primary combustion chamberand burns the majority of the volatile gases (typically methane andmethanol), tars and creosotes expelled from the burning wood. Tertiarycombustion of the remaining volatile gases takes place in the charcoalresting on the fine mesh charcoal basket which ultimately achieves atemperature of approximately 1,000° C.

Although the invention has been described with reference to a specificexample, it will be appreciated by persons skilled in the art that theinvention may be embodied in other forms within the broad scope andambit of the invention as herein set forth and defined by the followingclaims.

1. A wood fuelled heating stove including: a stove chamber having afloor, a side wall extending upwardly from the periphery of the floorand a top wall extending across the top of the side wall; a primarycombustion chamber at least a part of which extends at least part wayinto the stove chamber through an opening penetrating the top wall, theprimary combustion chamber having a base wall interposed between thefloor and the top wall of the stove chamber and having a primary gratingprovided comprising at least a part of the base wall, a curtain wallextending upwardly from the base wall through the opening in the topwall and having a plurality of ports penetrating the curtain wall and aclosure wall extending across the top of the curtain wall to enclose theprimary combustion chamber; an air jacket wall circumscribing at leastpart of the curtain wall at least partly below the opening to provide anair jacket circumscribing the primary combustion chamber but separatedtherefrom by the curtain wall, the curtain wall having a plurality ofports distributed about and penetrating therethrough to provide fluidcommunication between the air jacket and the primary combustion chamber,the air jacket wall and/or the curtain wall substantially sealing theopening through the top wall and separating the air jacket from thestove chamber; an air inlet opening through the floor or the side wallat or near the floor and a stove chamber air inlet into the stovechamber through or near to the top wall; a primary air duct, a secondaryair duct and a tertiary air duct the primary air duct extending alongthe floor of the stove chamber and providing fluid communication fromthe air inlet to the secondary air duct, the secondary air ductextending upwardly from the primary air duct and the tertiary air ductproviding fluid communication between the secondary air duct and the airjacket and the stove chamber air inlet; a charcoal grating partitioninterposed between the floor of the stove chamber and the base wall ofthe primary combustion chamber; and a flue passage extending from a flueopening to the stove chamber lower than the grating wall and upwardly toa flue outlet higher than the top and closure walls.
 2. A wood fuelledheating stove including: a stove portion having a stove chamber thereinand an fine mesh charcoal basket dividing the stove chamber into upperand lower chambers; a reduction portion having a primary combustionchamber therein and at least partly within the upper chamber of thestove chamber and having a primary grating vertically separating theprimary combustion chamber from the upper chamber; an air jacketsurrounding at least some of the primary combustion chamber and having aplurality of ports providing fluid communication between the primarycombustion chamber and the air jacket and a primary grating providinglimited flow of mobile solids and fluid between the primary combustionchamber and the stove chamber; an air passage providing fluidcommunication from the outside of the stove chamber to the air jacketand to the inside of the stove chamber; and a flue passage providingfluid communication from the lower chamber of the stove chamber to aflue outlet at a level above the primary combustion chamber and upperlevel of the stove chamber.
 3. The wood fuelled heating stove accordingto claim 2, wherein a portion of the grating wall comprises a gratingthrough which ash or such like may fall to rest upon the floor of thestove chamber.
 4. The wood fuelled heating stove according to claim 1,wherein the side wall includes four side wall portions, eachsubstantially planar which, together with the floor and top wall,provide a substantially rectangular prism form for the outside walls ofthe stove chamber, one of the side wall portions having a door openingthrough and a door being moveable between an open attitude permittingthe access to the stove chamber and a closed attitude substantiallysealing the stove chamber against air flow through the door opening aswell as sealing the upper chamber from the lower chamber.
 5. The woodfuelled heating stove according to claim 4, wherein the air jacketsurrounds the lower portion of the primary combustion chamber inrectangular prism form, part of the air chamber wall being provided asan extension of the base wall or extending outwardly from the chamberwall.
 6. The wood fuelled heating stove according to claim 4, whereinthe air jacket wall extends upwardly from its attachment to the basewall or curtain wall to the inside edges of the opening through the topwall of the stove chamber.
 7. The wood fuelled heating stove accordingto claim 4, wherein the air jacket wall ends short of the top wall andrejoins the curtain wall.
 8. The wood fuelled heating stove according toclaim 1, wherein the air inlet to the primary air duct is providedsubstantially centrally through the front wall of the stove below thedoor such that the primary air duct extends horizontally along the floorand front of the stove chamber to the secondary air duct, whichcomprises two rising ducts which run along each side of the front wallfrom the ends of the primary air duct.
 9. The wood fuelled heating stoveaccording to claim 8, wherein the primary air duct has a damperinterposed between the air inlet and the secondary air duct for dampingor restricting airflow through the primary air duct.
 10. The woodfuelled heating stove according to claim 1, wherein the plurality ofports are arranged to provide limited air flow to the primary combustionchamber, but the majority of the air pressure drop thereto.
 11. The woodfuelled heating stove according to claim 1 which, in use, provides areverse flow, progressive oxidation wood fuel heating stove encompassingan improved combustion process.
 12. The wood fuelled heating stoveaccording to claim 11, wherein oxidation of wood fuel in the stove isprogressive, the wood moving by gravity to locations of increasingoxidation with a gas and flame flow which is the reverse of that whichmay be regarded as “normal”.
 13. The wood fuelled heating stoveaccording to claim 2, wherein the plurality of ports are arranged toprovide limited air flow to the primary combustion chamber, but themajority of the air pressure drop thereto.
 14. The wood fuelled heatingstove according to claim 2 which, in use, provides a reverse flow,progressive oxidation wood fuel heating stove encompassing an improvedcombustion process.